Flocculation, Gravity Flows, and Total Organic Carbon Hotspots in Lake: Insights From Flume Experiments

Lakes serve as one of the significant sinks for organic carbon. For lake deposits, it is generally accepted that water depth is a primary control on the spatial distribution of total organic carbon (TOC) accumulation because the deeper part of a lake potentially has a higher organic population to be settled. However, lake TOC distribution is often spatially variable regardless of water depth, and an exact mechanistic explanation of TOC hotspots that do not align with the deepest water depth remaines unsolved. We suggest that flocculation significantly influences the lake TOC distribution pattern. The flocculation of fine sediment is possibly a major contributor to the occurrence of high TOC in deposits, as organic material aggregates with flocculated sediment during settling. The process of flocculation positively correlates with water salinity and thus potentially enriches TOC in deposits under a high saline condition. Furthermore, depending on the extent of flocculation, a lake entering plume type is different, leading to different run-out distances. Therefore, to investigate the role of flocculation associated with the TOC pattern in a lake, we design settling and gravity flow experiments with varying salinity and sediment concentration. Considering the flocculated grain size is seldom recorded in deposits, the current study provides a possible mechanism for why sediments with similar grain size deposited at similar water depths could have varying TOC content. This leads to a better understanding of the spatial distribution of carbon storage and the formation of source rocks associated with hydrocarbon reservoirs.